WHAT A RELIEF!: Miguel Batista, who gave up two runs last night, will get a chance to stay on the team as a starter. If he performs poorly Saturday, prospect Matt Harvey could take his place. Photo: Jeff Zelevansky

The Mets’ reluctance to lose Miguel Batista was a strong factor in the selection of the veteran righty over prospect Matt Harvey for Saturday’s start against the Dodgers, a team source said yesterday.

If Harvey was selected, the Mets in all likelihood would have cleared roster space by exposing Batista to waivers. It could have left the Mets without a backup plan if Harvey came up, was unsuccessful and needed a return to Triple-A Buffalo.

Instead, the decision was reached to let the 41-year-old Batista, who has struggled as a reliever, make his case to remain with the team by getting a shot as a starter — a role in which he has succeeded in limited opportunities over the last two seasons.

Last night Batista allowed two runs in the seventh inning as a reliever in the Mets’ 4-3 loss to the Nationals. Batista was hit in the left leg by a Roger Bernadina line drive, but said the ball only grazed him and he is fine for Saturday.

Batista seems to realize his leash is short.

“I’m getting a start Saturday,” Batista told The Post before last night’s game. “That is the only thing I can be sure of.”

* Jon Rauch wasn’t available to pitch Tuesday after having both knees drained of fluid, but threw a scoreless eighth inning last night. Manager Terry Collins said Rauch had the knees drained before the All-Star break, but recently had a recurrence in swelling.

* Frank Francisco likely is at least two weeks away from rejoining the Mets. The closer, who is on the disabled list with a strained left oblique, won’t begin throwing from a mound until Monday after a recent setback. The plan then calls for Francisco to begin pitching minor league rehab games a week from today. The righty will need two or three minor league games before returning to the Mets.

* Jason Bay received the left-field assignment over Jordany Valdespin, according to Collins, so the Mets could have a better defensive player with Chris Young on the mound. Young is a fly ball pitcher.

But it’s clear Bay will have to hit if he’s going to remain an everyday player, as Collins will look for Valdespin and Kirk Nieuwenhuis to receive at-bats.